Improving Drop Size and Velocity Estimates of an Optical Disdrometer: Implications for Sprinkler Irrigation Simulation

Optical disdrometers measure the attenuation of an infrared beam when water drops pass between the emitter and the receptor. The duration and intensity of the attenuation are used to estimate drop size and time of passage. These variables are used to calibrate and validate ballistic sprinkler simulation models. Two experimental problems affect the quality of the measurements: first, drops can pass to the side of the detector, so that only part of the drop attenuates the luminous flow; and second, several drops can overlap as they pass through the beam. This work presents a statistical treatment of the observed time of passage that can be used to eliminate a large part of the erroneous measurements, significantly improving the accuracy of disdrometer data. Furthermore, drop velocities can be estimated from the corrected times of passage. Simulation with the ballistic model shows that the minimum drop size accurately measured by the disdrometer is too large to characterize the fine diameters typical of drops landing close to the emitter. For farther landing distances, the discrepancies between measurements and simulations using ballistic theory can be large. Differences in drop velocity, drop size, and maximum sprinkler reach are discussed in this article. From our results, it can be concluded that the ballistic model (assuming independent movement of drops) constitutes an excessive simplification of reality. We believe that group displacement of the drops, resulting in a reduced air drag and in an increased probability of drop collision, is responsible for a relevant part of the reported differences.